Instruments such as x-ray detection devices and scanning electron microscopes make use of scintillation crystals to receive and detect high energy particles, such as secondary electrons. The particular case of scanning electron microscopes will be generally used throughout this disclosure, but it is appreciated that this is by way of example only, and that other devices that make use of scintillation detection channels are also comprehended hereunder.
A scanning electron microscope operates by directing a beam of electrons onto the surface of an object under investigation, such as a substrate upon which integrated circuits are fabricated. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
This primary beam of electrons is rastered across the surface of the object, and as it does so, so-called secondary electrons are emitted from the surface at characteristic angles and energies. These secondary electrons are received by one or more detection devices of some sort, which—in correlation with the scan pattern of the primary beam—are interpreted as various characteristics of the sample.
One such detector is comprised of one or more crystals of a material that scintillates when it is struck by an electron, such as yttrium aluminum perovskite, commonly referred to as YAP, or yttrium orthosilicate, commonly referred to as YSO, or any similar scintillating material. Thus, these scintillation crystals produce photons as they are struck by electrons. The photons are routed from the scintillation crystal to a photomultiplier tube via a light pipe. The photomultiplier tube receives the photons and multiplies—or in other words amplifies—the signal represented by the photons. This signal is received and interpreted by control and analysis electronics, and an image of the object is generated.
Unfortunately, there are many ways in which the primary signal, as represented by the original electron that strikes the scintillation crystal, can be diminished or lost. Further, there are many ways in which stray signals, otherwise known as noise, can be introduced into the signal path. Both of these effects tend to reduce the signal to noise ratio of the scintillation channel, which for the purposes of this disclosure is defined as those elements of an instrument from the scintillation crystal to the photomultiplier tube.
For example, it is difficult for photons generated within the scintillator to escape across the back surface, even if this surface is roughened and a coupling epoxy is present. There is also cross-channel signal exchange between light and dark field signals, light loss due to the acceptance angle of the fiber bundle, losses due to packing of the fibers and their cladding, absorption of light by the fiber, and losses due to poor coupling from the end of the fiber to the window and the photomultiplier tube. In all, only a few percent of the light generated in the scintillator actually makes it to the photomultiplier tube.
Further, the low efficiency of current detectors, at only several percent, necessitates the use of high beam currents and long exposures, increasing sample contamination (burn/carryover) and reducing throughput. In addition, cross-channel interactions causes a reduction in separation between the various detectors, mixing the bright field and dark field images, and reducing the effective dark field angle of the detector.
What is needed, therefore, is a system by which problems such as those described above are reduced, at least in part.